As a method of separating and feeding stacked sheets, such as documents and recording sheets, there is an electrostatic attraction separation method of generating an electric field on an attraction belt and bringing the attraction belt into contact with a sheet to attract and separate the sheet from other sheets. A background sheet feeder according to the electrostatic attraction separation method includes an attraction separation unit which includes an attraction belt formed by a dielectric member wound around two rollers, a charger for supplying alternating charge to the attraction belt, and a holder holding the attraction belt and the charger. The holder rotatably supports the two rollers, and is fixed to a rotary shaft provided upstream of the two rollers in the sheet feeding direction. One end of the rotary shaft is provided with a gear mechanism including a rack and pinion and a swing mechanism serving as a swing device for swinging the attraction separation unit. Further, one of the two rollers disposed on the upstream side in the sheet feeding direction (hereinafter referred to as upstream tension roller) includes a shaft inserted in slots in the holder and supported by the holder to be movable in a direction perpendicular to the sheet surface of a sheet bundle.
Prior to the sheet feeding operation, the attraction belt held by the holder via the two rollers is located at a position separated from the sheet bundle. In this state, the upstream tension roller is in contact with the respective lower ends of the slots, and is closer to the sheet bundle than the other roller disposed on the downstream side in the sheet feeding direction (hereinafter referred to as downstream tension roller) is.
To feed the uppermost sheet of the sheet bundle by separating the sheet from the other sheets, the attraction belt is first rotated and supplied with alternating charge. After the supply of the alternating charge to the attraction belt, the rotation of the attraction belt is stopped. Then, the swing mechanism is driven to swing the attraction separation unit toward the sheet bundle. According to this action, the upstream tension roller first comes into contact with the uppermost sheet of the sheet bundle via the attraction belt. If the attraction separation unit in this state is further swung, the shaft of the upstream tension roller moves within the slots in the holder and comes into contact with the respective upper ends of the slots. At the same time, the downstream tension roller comes into contact with the uppermost sheet of the sheet bundle via the attraction belt. According to this action, an area of the attraction belt facing the sheet bundle is brought into contact with the uppermost sheet, and the uppermost sheet of the sheet bundle is attracted to the attraction belt. After the attraction of the uppermost sheet of the sheet bundle to the attraction belt, the swing mechanism is driven to swing the attraction separation unit in a direction separating from the sheet bundle. According to this action, the downstream tension roller is separated from the sheet bundle together with the holder. By contrast, the upstream tension roller remains on the sheet bundle, and the shaft of the upstream tension roller relatively moves within the slots. If the attraction separation unit is further swung in the direction separating from the sheet bundle, the shaft of the upstream tension roller comes into contact with the lower ends of the slots, and the upstream tension roller is separated from the sheet bundle while being supported by the lower ends of the slots. According to this action, the uppermost sheet attracted to the attraction belt is lifted by the attraction belt and separated from a second sheet. Then, the attraction belt is driven to rotate and feed the uppermost sheet attracted thereto.
To reliably bring the uppermost sheet of the sheet bundle and the attraction belt into contact with each other, the background sheet feeder includes a contact state detection device which detects the contact of the attraction belt with the sheet bundle. Specifically, the contact state detection device is provided to the upper ends of the slots. As described above, when the area of the attraction belt facing the sheet bundle comes into contact with the uppermost sheet of the sheet bundle, the shaft of the upstream tension roller comes into contact with the upper ends of the slots. Thus, the contact state detection device is provided at the upper ends of the slots. On the basis of the contact of the shaft of the upstream tension roller with the contact state detection device, therefore, the contact of the area of the attraction belt facing the sheet bundle with the uppermost sheet is detected. Then, upon detection by the contact state detection device of the contact of the attraction belt with the uppermost sheet, the swing of the attraction separation unit is stopped. With this configuration, the attraction belt is expected to be reliably brought into contact with the uppermost sheet.
In the background sheet feeder, however, the provision of the contact state detection device results in an increase in the number of components and the cost of the sheet feeder. Further, the driving of the swing device is controlled on the basis of the result of detection by the contact state detection device, and thus the drive control is complicated. Further, a bottom plate loaded with the sheet bundle is configured to be movable in a direction perpendicular to the sheet surface of the sheet bundle. When the bottom plate is lifted to move the uppermost sheet to a predetermined position, if the movement amount of a downstream end portion of the bottom plate in the sheet feeding direction moved by a drive device is less than the movement amount of an upstream end portion of the bottom plate in the sheet feeding direction, the sheet bundle loaded on the bottom plate may be inclined. Such an inclination may cause the downstream side of the sheet bundle in the sheet feeding direction to be more distant from the attraction belt than the upstream side of the sheet bundle in the sheet feeding direction is. In this case, even if the shaft of the upstream tension roller comes into contact with the contact state detection device and the contact state detection device detects the contact, the downstream side of the attraction belt in the sheet feeding direction may be separated from the uppermost sheet, and the area of the attraction belt facing the sheet bundle may fail to come into contact with the uppermost sheet of the sheet bundle.